In an electrophotographic image forming apparatus such as a laser printer, an electrostatic copying machine, a plain paper facsimile machine or a printer-copier-facsimile multifunction machine, an image is generally formed on a surface of a sheet such as a paper sheet or a plastic film through the following process steps.
First, a surface of a photoreceptor body having photoelectric conductivity is evenly electrically charged and, in this state, exposed to light, whereby an electrostatic latent image corresponding to an image to be formed on the sheet is formed on the surface of the photoreceptor body (charging step and exposing step).
Then, toner (minute color particles) preliminarily electrically charged at a predetermined potential is brought into contact with the surface of the photoreceptor body. Thus, the toner selectively adheres to the surface of the photoreceptor body according to the potential pattern of the electrostatic latent image, whereby the electrostatic latent image is developed into a toner image (developing step).
Subsequently, the toner image is transferred onto the surface of the sheet (transfer step), and fixed to the surface of the sheet (fixing step). Thus, the image is formed on the surface of the sheet.
Further, a part of the toner remaining on the surface of the photoreceptor body after the transfer of the toner image is removed, for example by a cleaning blade or the like (cleaning step). Thus, the photoreceptor body is ready for the next image formation.
In the charging step out of the aforementioned process steps, a charging roller is used, which is kept in contact with the surface of the photoreceptor body to evenly electrically charge the surface of the photoreceptor body.
Widely used as the charging roller is an electroconductive roller having an outer peripheral surface to be kept in contact with the surface of the photoreceptor body, at least the outer peripheral surface being made of a crosslinking product of an electrically conductive rubber composition and having lower resistance.
Generally usable as the rubber composition for the electroconductive roller is a rubber composition imparted with ion conductivity by blending a rubber component including at least an ion conductive rubber (e.g., an epichlorohydrin rubber) and a diene rubber, a crosslinking component for crosslinking the rubber component, and the like.
In order to further reduce the resistance of the electroconductive roller, a slat of an anion containing a fluoro group and a sulfonyl group and a cation of a metal element, or an ionic liquid is often blended in the rubber composition (Patent Document land the like), or electrically conductive carbon is blended in the rubber composition.